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      <div class="post-title">Spring Boot自动配置原理</div>
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          2021-07-31 15:46:24
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                  <a href="/categories/Spring-Boot/" title="Spring Boot">
                    <b>#</b> Spring Boot
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        <p>我们知道，Spring Boot 项目创建完成后，即使不进行任何的配置，也能够顺利地运行，这都要归功于 Spring Boot 的自动化配置。</p>
<p>Spring Boot 默认使用 application.properties 或 application.yml 作为其全局配置文件，我们可以在该配置文件中对各种自动配置属性（server.port、logging.level.* 、spring.config.active.no-profile 等等）进行修改，并使之生效，那么您有没有想过这些属性是否有据可依呢？答案是肯定的。</p>
<p>Spring Boot 官方文档：常见应用属性中对所有的配置属性都进行了列举和解释，我们可以根据官方文档对 Spring Boot 进行配置，但 Spring Boot 中的配置属性数量庞大，仅仅依靠官方文档进行配置也十分麻烦。我们只有了解了 Spring Boot 自动配置的原理，才能更加轻松熟练地对 Spirng Boot 进行配置。本节为你揭开 SpringBoot 自动配置的神秘面纱。</p>
<h1 id="Spring-Factories机制"><a href="#Spring-Factories机制" class="headerlink" title="Spring Factories机制"></a>Spring Factories机制</h1><p>Spring Boot 的自动配置是基于 Spring Factories 机制实现的。</p>
<p>Spring Factories 机制是 Spring Boot 中的一种服务发现机制，这种扩展机制与 Java SPI 机制十分相似。Spring Boot 会自动扫描所有 Jar 包类路径下 META-INF&#x2F;spring.factories 文件，并读取其中的内容，进行实例化，这种机制也是 Spring Boot Starter 的基础。</p>
<h1 id="SpringApplication这个类主要做了以下几件事情"><a href="#SpringApplication这个类主要做了以下几件事情" class="headerlink" title="SpringApplication这个类主要做了以下几件事情"></a>SpringApplication这个类主要做了以下几件事情</h1><ol>
<li>推断应用是普通项目还是web项目</li>
<li>查找并加载初始化器，设置到initializers中</li>
<li>找到所有的应用程序监听器，设置到listers中</li>
<li>推断并设置main方法的定义类，找到运行的主类</li>
</ol>
<h1 id="Spring-Boot的配置文件"><a href="#Spring-Boot的配置文件" class="headerlink" title="Spring Boot的配置文件"></a>Spring Boot的配置文件</h1><p>初识Spring Boot时我们就知道，Spring Boot有一个全局配置文件：application.properties或application.yml。</p>
<p>我们的各种属性都可以在这个文件中进行配置，最常配置的比如：server.port、logging.level.* 等等，然而我们实际用到的往往只是很少的一部分，那么这些属性是否有据可依呢？答案当然是肯定的，这些属性都可以在官方文档中查找到：</p>
<p><a target="_blank" rel="noopener" href="https://docs.spring.io/spring-boot/docs/2.1.0.RELEASE/reference/htmlsingle/#common-application-properties">https://docs.spring.io/spring-boot/docs/2.1.0.RELEASE/reference/htmlsingle/#common-application-properties</a></p>
<h1 id="工作原理剖析"><a href="#工作原理剖析" class="headerlink" title="工作原理剖析"></a>工作原理剖析</h1><p>Spring Boot的启动类上有一个@SpringBootApplication注解，这个注解是Spring Boot项目必不可少的注解。那么自动配置原理一定和这个注解有着千丝万缕的联系！</p>
<h2 id="EnableAutoConfiguration"><a href="#EnableAutoConfiguration" class="headerlink" title="@EnableAutoConfiguration"></a>@EnableAutoConfiguration</h2><p> @SpringBootApplication是一个复合注解或派生注解，在@SpringBootApplication中有一个注解@EnableAutoConfiguration，翻译成人话就是开启自动配置，其定义如下：</p>
<p><img src="http://qiniu.c77544s.top/picgo/202210121456386.png" alt="image-20210907163302698"></p>
<p>@EnableAutoConfiguration这个注解也是一个派生注解，其中的关键功能由@Import提供，其导入的AutoConfigurationImportSelector的<code>selectImports()</code>方法通过<code>SpringFactoriesLoader.loadFactoryNames()</code>扫描所有具有META-INF&#x2F;spring.factories的jar包。spring-boot-autoconfigure-x.x.x.x.jar里就有一个这样的spring.factories文件。</p>
<p>这个spring.factories文件也是一组一组的key&#x3D;value的形式，其中一个key是EnableAutoConfiguration类的全类名，而它的value是一个xxxxAutoConfiguration的类名的列表，这些类名以逗号分隔，如下图所示：</p>
<p><img src="http://qiniu.c77544s.top/picgo/202210121456387.png" alt="image-20210907163615424"></p>
<p>这个@EnableAutoConfiguration注解通过@SpringBootApplication被间接的标记在了Spring Boot的启动类上。在SpringApplication.run(…)的内部就会执行selectImports()方法，找到所有JavaConfig自动配置类的全限定名对应的class，然后将所有自动配置类加载到Spring容器中。</p>
<h1 id="自动配置生效"><a href="#自动配置生效" class="headerlink" title="自动配置生效"></a>自动配置生效</h1><p>每一个XxxxAutoConfiguration自动配置类都是在某些条件之下才会生效的，这些条件的限制在Spring Boot中以注解的形式体现，常见的条件注解有如下几项：</p>
<blockquote>
<p>@ConditionalOnBean：当容器里有指定的bean的条件下。</p>
<p>@ConditionalOnMissingBean：当容器里不存在指定bean的条件下。</p>
<p>@ConditionalOnClass：当类路径下有指定类的条件下。</p>
<p>@ConditionalOnMissingClass：当类路径下不存在指定类的条件下。</p>
<p>@ConditionalOnProperty：指定的属性是否有指定的值，比如@ConditionalOnProperties(prefix&#x3D;”xxx.xxx”, value&#x3D;”enable”, matchIfMissing&#x3D;true)，代表当xxx.xxx为enable时条件的布尔值为true，如果没有设置的情况下也为true。</p>
</blockquote>
<p><a target="_blank" rel="noopener" href="http://c.biancheng.net/spring_boot/auto-config.html">具体内容点击我</a></p>

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        <ol class="toc"><li class="toc-item toc-level-1"><a class="toc-link" href="#Spring-Factories%E6%9C%BA%E5%88%B6"><span class="toc-text">Spring Factories机制</span></a></li><li class="toc-item toc-level-1"><a class="toc-link" href="#SpringApplication%E8%BF%99%E4%B8%AA%E7%B1%BB%E4%B8%BB%E8%A6%81%E5%81%9A%E4%BA%86%E4%BB%A5%E4%B8%8B%E5%87%A0%E4%BB%B6%E4%BA%8B%E6%83%85"><span class="toc-text">SpringApplication这个类主要做了以下几件事情</span></a></li><li class="toc-item toc-level-1"><a class="toc-link" href="#Spring-Boot%E7%9A%84%E9%85%8D%E7%BD%AE%E6%96%87%E4%BB%B6"><span class="toc-text">Spring Boot的配置文件</span></a></li><li class="toc-item toc-level-1"><a class="toc-link" href="#%E5%B7%A5%E4%BD%9C%E5%8E%9F%E7%90%86%E5%89%96%E6%9E%90"><span class="toc-text">工作原理剖析</span></a><ol class="toc-child"><li class="toc-item toc-level-2"><a class="toc-link" href="#EnableAutoConfiguration"><span class="toc-text">@EnableAutoConfiguration</span></a></li></ol></li><li class="toc-item toc-level-1"><a class="toc-link" href="#%E8%87%AA%E5%8A%A8%E9%85%8D%E7%BD%AE%E7%94%9F%E6%95%88"><span class="toc-text">自动配置生效</span></a></li></ol>
      
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